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Marine and Petroleum Geology xxx (2009) 1–12

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Marine and Petroleum Geology

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Triangulation-to-GPS and GPS-to-GPS geodesy in Trinidad, West Indies:Neotectonics, seismic risk, and geologic implications

J.C. Weber a,*, J. Saleh b,1, S. Balkaransingh b,2, T. Dixon c, W. Ambeh d,3,T. Leong e, A. Rodriguez a, K. Miller b,4

a Department of Geology, Grand Valley State University, 1 Campus Drive, Allendale, MI 49401, USAb Department of Surveying and Land Information, University of the West Indies, St. Augustine, Trinidad and Tobagoc Rosensteil School of Marine and Atmospheric Sciences, University of Miami, Miami, FL 33149, USAd Seismic Research Unit, University of the West Indies, St. Augustine, Trinidad and Tobagoe Trinidad and Tobago Lands and Surveys Division, 118 Frederick Street, Port-of-Spain, Trinidad and Tobago

a r t i c l e i n f o

Article history:Received 17 November 2008Received in revised form2 July 2009Accepted 23 July 2009Available online xxx

Keywords:GPSGeodesyTriangulationNeotectonicsTrinidadVenezuelaCentral Range FaultElastic locking

* Corresponding author. Tel.: þ1 616 331 3191; fax:E-mail address: [email protected] (J.C. Weber).

1 Present address: National Geodetic Survey, Silver2 Present address: BpTT, Port-of-Spain, Trinidad an3 Present address: P.O. Box 5014, Nkwen-Bamenda4 Present address: The British University in Egypt,

0264-8172/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.marpetgeo.2009.07.010

Please cite this article in press as: Weber, J.Marine and Petroleum Geology (2009), doi:

a b s t r a c t

We estimated horizontal velocities at 25 sites first surveyed in a 1901–1903 British Ordnance Surveytriangulation and then resurveyed with GPS in 1994–1995 to identify Trinidad’s principal active on-landfaults, quantify fault-slip-rates, and test for elastic locking. Our best-fit single-fault elastic dislocationmodel put 12 � 3 mm/yr of dextral strike-slip on the Central Range Fault (1–2 km locking depth), anapparently aseismic active fault. The estimated motions also showed statistically insignificant horizontalmotion (2.2 � 1.8 mm/yr of right-slip; 2.7 � 2.0 mm/yr of N–S shortening) on the eastward on-strikeextension of the El Pilar Fault, known to be the active transform fault in Venezuela. Repeat GPSmeasurements made between 1994 and 2005 at two sites spanning the island north to south showeda 14 � 3 mm/yr eastward (plate-motion-parallel) dextral velocity differential, consistent with our best-fithistoric (1901–1995) fault-slip-rate. Paleoseismology trenching demonstrates that the Central RangeFault cuts <5000-year-old sediment and is capped by w550-year-old sediment, suggesting that it may belocked and may have ruptured at least once during this time interval. About w5 mm/yr of slip could betaken up on the Los Bajos Fault and additional faults in the offshore south of Trinidad. The existing1901–1995 and 1994–2005 geodetic data alone cannot resolve whether the Central Range Fault isessentially creeping (�1–2 km locking depth) or locked to a more standard depth of 10 km.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Trinidad and Tobago is a two-island nation in the southeasterncorner of the Caribbean that sits in the geologically complex,actively deforming, Caribbean–South American plate boundaryzone. Trinidad is a densely populated (w1 millionþ citizens), majorregional industrial center that produces oil and gas onshore andoffshore, exports oil, gas, and LNG, and is covered by a largepetroleum infrastructure (pipelines, national refinery, LNG plant).The purpose of this contribution is to lay out the motivation,methods, findings, and implications of our decade-long program to

þ1 616 331 3740.

Spring, MD 20910, USA.d Tobago., NW Province, Cameroon.El Sherouk, Egypt.

All rights reserved.

C., et al., Triangulation-to-GP10.1016/j.marpetgeo.2009.07.

study active deformation (neotectonics) in Trinidad using geodesy.Part of this work has been previously published (Weber et al.,2001a; Saleh et al., 2004). Here, we present the unpublished (e.g.,elastic dislocation fault modeling) part of the work for the firsttime. We update Weber et al.’s (2001a) Trinidad GPS-to-GPSvelocities by including new 2005 data for two sites, and using moredata from more sites to define a South American reference frame.We summarize the published work, synthesize and interpret all ofthe data, present paleoseismic and tectonic geomorphic evidencethat bear on the geodetic results, and discuss implications forTrinidad’s petroleum infrastructure, structural trap development,and paleotectonic history.

We began this work with the goal of trying to identify Trinidad’sprincipal active faults, quantify their slip rates, and test for elasticfault locking. Geodesy provided a direct approach to resolve theseissues, and before our work, had not previously been applied inTrinidad. Trinidad was formerly a British colony, and containsa historic triangulation network that was built and first measured

S and GPS-to-GPS geodesy in Trinidad, West Indies: Neotectonics,...,010

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J.C. Weber et al. / Marine and Petroleum Geology xxx (2009) 1–122

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almost a century ago (1901–1903). Starting in 1993, we begangathering Trinidad’s historic triangulation data and planninga modern GPS (Global Positioning System) reoccupation of thehistoric network. In 1994–1995, we recovered and resurveyed withGPS (Global Positioning System) 27 primary and secondary stationsin this historic network, readjusted the original data, and began tocompare relative positions between the two surveys. This allowedus to resolve century-time-scale velocities, to identify the CentralRange Fault as the principal active fault, and to quantify a historicslip rate across it (12 � 3 mm/yr). Statistically insignificant hori-zontal motion (2.2 � 1.8 mm/yr) occurred over this time periodacross the El Pilar Fault’s eastward on-strike extension intoTrinidad. Repeat GPS measurements made in 1994, 1998, 1999, and2005 at two monuments spanning the island from north to southshowed 14 � 3 mm/yr of eastward (dextral) differential velocityacross Trinidad, corroborating both that the Central Range Fault isthe principal active fault and that our historic (1901–1995) fault-slip-rate estimate across it is accurate. Recent on-land paleo-seismology (Crosby et al., submitted for publication; Prentice et al.,submitted for publication) and tectonic geomorphology research(Weber and Ritter, 2007; Ritter and Weber, 2007), and analysis ofoffshore reflection seismic surveys (Soto et al., 2007) support thegeodetic results we present here. Combined together, these datahelp explain how the local movement and fault geometry wequantify in Trinidad fits into a broader context of Caribbean–SouthAmerican plate tectonics and Gulf of Paria pull-apart tectonics. Inaddition to the obvious contribution of addressing seismic risk andcoastal subsidence in Trinidad, which is vital for planning a durablepetroleum infrastructure, having a clearer understanding of theneotectonics also allows one to ‘‘strip off’’ recent deformation andgain a clearer understanding of Trinidad’s complex structural trapdevelopment and paleotectonic history. These are critical elementsin understanding Trinidad’s petroleum system.

2. Plate tectonic setting and active plate boundary zonedeformation

Weber et al. (2001a), Perez et al. (2001), Sella et al. (2002), Lopezet al. (2006), and Kreemer et al. (2003) presented direct and preciseGPS estimates of current Caribbean–South American plate motion.Weber et al. (2001a) used a mix of continuous and campaign-styleGPS data at five widely spaced South American sites and eightwidely spaced Caribbean sites. Site velocities on each plate were fitto respective rigid plate models, yielding w1.5 mm/yr rate resid-uals, approximately equal to the level of uncertainty in the GPSvelocity estimates. The Caribbean–South American plate motiondirection predicted by the Weber et al. (2001a) model is close inazimuth to the NUVEL-1 ‘‘pure strike-slip’’ model (DeMets et al.,1990), but gives a rate that is almost twice as fast. The Weber et al.(2001a) model predicts 20 � 3 mm/yr of N86 � 2�E motion inTrinidad, and 20 � 3 mm/yr of motion toward 090 � 2� on the ElPilar Fault in Venezuela. These predicted directions agree reason-ably well with earthquake slip vectors from the seismically activeVenezuelan (El Pilar) portion of the plate boundary zone (seeWeber et al., 2001a, Fig. 2). Perez et al. (2001) used GPS data mostlyfrom sites in Venezuela. These sites involved some complicationsrelated to removing the coseismic displacements from the 1997 Ms

6.8 El Pilar event and were referenced to the South American plateby using a single fixed South American site. Nonetheless, Perez et al.(2001) predicted comparable 20.5 mm/yr Caribbean–South Amer-ican plate motion rates with directions that are slightly morenortherly than those of Weber et al. (2001a) in Trinidad. Sella et al.(2002), Kreemer et al. (2003), and Lopez et al. (2006) all build onand incrementally improved the results from the Weber et al.(2001a) study. These GPS studies provide important kinematic

Please cite this article in press as: Weber, J.C., et al., Triangulation-to-GPMarine and Petroleum Geology (2009), doi:10.1016/j.marpetgeo.2009.07

boundary conditions, placing an upper bound on the total motionpossible across the Trinidad study area, and predicting that trans-form (strike-slip) neotectonics should be occurring there.

The GPS work discussed above was motivated mostly by the lackof geologic data of the type conventionally used to measure platemotion, such as spreading rates and transform fault azimuths, inthe Caribbean–South American plate boundary zone. As a result,Caribbean–South American relative plate motion was very poorlyresolved prior to the direct GPS estimates discussed above. Bypropagating conventional plate motion data from other platesthrough a global plate motion circuit, DeMets et al. (1990, 1994)predicted that 12.7 � 3 mm/yr of N85� � 10�E directed Caribbean–South American motion occurs near Trinidad, suggesting thatactive dextral strike-slip occurs in the Caribbean–South Americanplate boundary zone. An alternative prediction of 12.1�3 mm/yr ofS68 � 10�E directed motion can be obtained by scaling the alter-native Caribbean–South American Euler pole of DeMets et al.(1990) (derived without using any Lesser Antilles slip vectors) withthe NUVEL-1A rate reduction factor (DeMets et al., 1994). Thissecond estimate suggested that transpression may be the currenttectonic style in Trinidad. A third possibility came from Deng andSykes (1995), who estimated a Caribbean–South American pole ofrotation using only earthquake slip vectors, the Caribbean platemotion circuit, and vector subtraction, and suggested that trans-tension may be the current tectonic style in Trinidad. Discrepanciesbetween the geologic and seismological plate motion models andthe more recent GPS-derived models appear to result from biases inchoosing how to treat the small geologic and earthquake data sets.

Moderately sized earthquakes occur frequently on the El PilarFault in the Caribbean–South American plate boundary zone inVenezuela, west of Trinidad (Fig. 2). Mendoza (2000) discussed theclear progression and history of the 1929 M 6.9, 1968 M 6.2, 1974M 6.1, 1986 M 6.2, and 1997 M 6.1 events. Focal mechanisms fromthese events and a few others like them (e.g., pre-instrumentalevents; Doser and Van Dusen, 1996) clearly show that E–W dextralstrike-slip is taken up across Venezuela’s El Pilar Fault (Russo et al.,1993; Deng and Sykes, 1995; Mendoza, 2000; Weber et al., 2001a,Fig. 2). GPS results confirm that the El Pilar Fault is indeed theprincipal transform fault that takes up most of the current platemotion in eastern Venezuela (Perez et al., 2001).

Trinidad has a long recorded history of felt earthquakes thatdates back to w1800. This record indicates that a few large onshorehistoric events may have occurred, but it does not require any suchevents (Robson, 1964). Furthermore, shallow instrumental earth-quakes from local seismic networks show no clear linear clusteringof shallow earthquakes in Trinidad (Fig. 2). Using seismology alone,it has thus proven difficult to identify the location of Trinidad’sactive faults. This puzzle prompted a wide array of possiblescenarios (see below). In addition to addressing the active faultlocation question, geodesy also provided a tool to directly measurefault-slip-rates, and to begin to address whether Trinidad’s activefaults creep aseismically or are locked with the potential togenerate large earthquakes.

Using geologic data, Robertson and Burke (1989) and manyothers have suggested that the El Pilar transform fault in Venezuelacontinues on-strike to the east, onto and across the island of Tri-nidad. Speed (1985), Russo and Speed (1992), and Speed et al.(1991), objected to this interpretation. Speed (1985) and Russo andSpeed (1992) alternatively inferred an active north-dipping, south-verging thrust in this position in Trinidad, whereas Kugler’s (1960)geologic map of Trinidad shows both an exposed normal fault at thesouthern foot of the Northern Range (the Arima Fault; also seeWeber et al., 2001b), and a buried El Pilar continuation furthersouth in the Northern (Caroni) Basin (Fig. 3A). Algar and Pindell(1994), Flinch et al. (1999), and Babb and Mann (1999), used

S and GPS-to-GPS geodesy in Trinidad, West Indies: Neotectonics,...,.010

Fig. 1. Map showing Trinidad study area and active fault system in Caribbean–SouthAmerican plate boundary as proposed in this study. The Central Range Fault (CRF) andEl Pilar Fault (EPF) are the main strike-slip (transform) faults. They take up themajority of the w20 mm/yr of east–west dextral shear related to Caribbean andSouth American plate motion, and are linked across the Gulf of Paria pull-apart basin.The poorly defined eastern boundary of the Maracaibo block (microplate) is shown asa dashed line.

J.C. Weber et al. / Marine and Petroleum Geology xxx (2009) 1–12 3

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offshore industry seismic data to suggest that dextral motion alongthe active Venezuelan El Pilar transform is transferred southwardacross the Gulf of Paria pull-apart basin to the Warm Springs Fault,an active dextral strike-slip fault in Trinidad’s western offshore. vanAndel and Sachs (1964) were the first to map an active fault thatcuts Holocene sediment and the seafloor in this position. Weber

Fig. 2. Shallow (<50 km deep) seismicity from 1910 to 2002 in the southeastern Caribbeseismicity on the El Pilar strike-slip (transform) fault in Venezuela, and the apparent aseismsuggest is the main active strike-slip fault in Trinidad. Earthquake symbol is proportional tcircles.

Please cite this article in press as: Weber, J.C., et al., Triangulation-to-GPMarine and Petroleum Geology (2009), doi:10.1016/j.marpetgeo.2009.07.

et al. (2001a) referred to the previously unmapped onshore activestrike-slip segment of this transform system as the Central RangeFault (Fig. 1), and Soto et al. (2007) followed suit in their study ofthe eastern offshore. Kugler’s (1960) map showed the WarmSprings Fault coming onshore as a short, segmented, bedrock thrustclose to where we currently map the Central Range Fault (Fig. 3A)(Crosby et al., submitted for publication; Prentice et al., submittedfor publication).

3. Geodesy

3.1. 1901–1903 Triangulation

The first survey of Trinidad was a triangulation done by theBritish Ordnance Survey in 1901–1903. Smart (1903) provides anexcellent summary of this seminal survey. This network consistedof 18 primary stations that formed a great chain of triangles, andincluded more than 600 secondary stations. Many of these primaryand secondary stations (monuments) still exist today (Fig. 4).Observations included more than 22,000 horizontal directions, 2baseline lengths, and 2 astronomical azimuths. One of the twobaselines measured was about 2.5 km long and was observed 4times with repeatability of better than 2 cm. Data from the secondbaseline were not available. We extracted the 1901–1903 angle andbaseline observations from approximately twenty 95-year-oldhand-written field books that are housed at the Trinidad andTobago Lands and Surveys Division headquarters in the Red Housein Port-of-Spain, Trinidad and coded them into a digital database.Grant (1996) and King (1996) list the digitized and edited data set.Balkaransingh (2001) first applied and discussed the data read-justment procedure we describe and use below. Saleh et al. (2004)also discuss these data and readjustment methods, with a focus onimproving the national geodetic framework.

In our analysis and final readjustment, we exclude secondarystations that are connected to the network at less than 2 points.

an as observed by the Venezuelan local network (FUNVISIS). Note the high-level ofic nature of the Central Range Fault in Trinidad, which geodetic data (present study)

o magnitude. Pre-1960 earthquakes are shown as triangles, post-1960 earthquakes as


Fig. 3. A. Sun-shaded digital elevation model showing Trinidad’s principal geomorphic features, including the Northern and Central Ranges, Gulf of Paria, Columbus Channel, andCentral Range and Los Bajos Fault traces. Submergent topographic and geomorphic features in western Trinidad and separated from emergent ones in the east by tilting ‘‘hinge line’’(Weber and Ritter, 2007; Ritter and Weber, 2007). Additional geologic and geographic locations discussed in text are also shown (A: Arima Fault; P-a-P: Pointe-a-Pierre; ND: NavetDam; S: Samlalsingh trench site of Prentice et al., submitted for publication; V: Valencia). B. 1901–1995 velocities, with 20 � 3 mm/yr eastward added to each site velocity to putthem into Weber et al.’s (2001a) fixed South American reference frame, shown as black arrows with 1s error ellipses. 1994–2005 repeat GPS velocities are also shown as heavy blackarrows with 1s error ellipses for sites TDAD (trigonometric station 69) and LFAB (trigonometric station 115); note that these two sites also have 1901–1995 triangulation-to-GPS velocities that can be seen and used for comparison. The 14 � 3 mm/yr differential 1994–2005 eastward motion between the two repeat GPS sites supports our historic12 � 3 mm/yr Central Range Fault-parallel slip rate (also see Fig. 4). Trinidad’s major faults are also shown and labeled.


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This resulted in a network of 374 stations, including about 20,000horizontal direction observations. These observations wereoriginally taken in sets of at least 6, and then averaged, resulting in3356 averaged directions, which comprise our primary source of1901–1903 data. The astronomical azimuths and baselines wereexcluded in the final adjustment.

Outliers in the original data and data entry errors were detectedby examining triangle misclosures during preliminary adjustments.After these outliers were corrected or eliminated, 581 triangles hadmisclosures of better than 1000 with a root-mean-square misclosureof 4.400. Nineteen triangles had misclosures between 1000 and 1900.The RMS misclosure of the 581 triangles was used to estimateaverage observational uncertainties for angles, sa ¼ 2.5500 and


directions shz ¼ 1.800. The 19 triangles with the largest misclosureswere used to identify weak directions in the original data set, eachof which was down-weighted in proportion to the magnitude of thecorresponding misclosures in the final adjustment. The editedaveraged direction observations were then adjusted on the WGS84ellipsoid centered on ITRF96 (Sillard et al., 1998) using a standardleast-squares triangulation adjustment procedure, followingBomford (1980) and described below. A priori station coordinateswere estimated and used to reduce (project) the averaged editedobservations from Earth’s surface onto the UTM projection plane,and to initialize a least squares adjustment. Reduction of thecoordinates from Earth’s surface to the WGS84 ellipsoid wasneglected due to the small magnitude of the corrections involved


Fig. 3 (continued).


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(<0.100) relative to the uncertainties in the data. The only additionalpossible non-negligible reductions are those due to deflections ofthe vertical and horizontal directions. A complete set of deflectionsof the vertical is not available, so this correction was not computed.

Observation equations were constructed on the UTM plane fromthe projected observations, and the normal system was sequen-tially built. To define the origin, orientation and scale of the1901–1903 coordinate system, a subset of four reference stations inthe western and central Northern Range, defined using the iterativeprocess described below, were fixed to their 1994–1995 epochITRF96 coordinates. This particular selection of reference stationswas supported by preliminary adjustments and by some geologicalinsight, which suggests that the western and central NorthernRange may form a tilted, but complete, rigid crustal section (block),within which stations probably do not move horizontally relative toone another (e.g., Weber et al., 2001b). In the final adjustment, weobtain decimeter-level coordinate uncertainties for 748 stationcoordinates (RMS ¼ �29 cm); 260 range from �0 to 10 cm, 138range from �10 to 20 cm, 111 range from �20 to 30 cm, 116 rangefrom�30 to 40 cm, 77 range from�40 to 50 cm, 38 range from�50to 60 cm, and 8 are >60 cm. We use these relatively imprecise datato resolve accurate site velocities by trading first epoch positioncertainty for the wcentury-long time span between the twomeasurement epochs.

3.2. The 1994–1995 GPS network

In 1993 we recovered nine 1901–1903 primary triangulationmonuments in pristine condition (site numbers 8, 12, 69, 82, 115,


135, 141, 145, 167; Tables 1 and 2). Many additional damagedmonuments, and sites where monuments were missing andpresumably destroyed, were also visited during this time. Weresurveyed the nine recovered stations in May–June 1994 using 4Trimble-4000-SSE dual-frequency receivers. Data were collected intwo bursts, for about 12 h or more per day, and for at least 3–4 daysat each site.

In 1994–1995, the Trinidad and Tobago Division of Lands andSurveys measured a GPS network in Trinidad consisting of 342baselines between 89 stations. Twenty-two of these stations were1901–1903 primary and secondary triangulation monuments,including 4 of the 9 monuments discussed above (see Table 1).Baseline lengths ranged from 1.2 to 65 km; most baselines wereabout 10–20 km long, and about one fifth of the baselines exceeded30 km in length. Baselines were observed using a pair of dual-frequency Trimble-4000-SSE receivers. A typical baseline surveylasted for several hours using a 15-s data interval. Longer(40–60 km) baselines were observed for 3–6 h. Edwards (1999)describes the 1994–1995 Lands and Survey’s GPS network and dataset in additional detail.

The 1994–1995 GPS baselines from the two campaignsdescribed above were processed using broadcast orbits andTrimble’s commercial GPSurvey software. Baseline solutions wereof the ionosphere-free, fixed-ambiguity type, with the exception ofa few short (w1–4 km) baselines, which were of the L1 fixed type.Baselines including station 115 (La Fabiana) as a base station, forwhich precise ITRF96 coordinates were known a priori (e.g., seeSection 5 below), were processed first. The resulting stationcoordinates were then used iteratively to process the remaining


Fig. 4. Photograph showing example 1901 British Ordnance Survey primary triangu-lation monument in the Northern Range, Trinidad (Cangrejal, site #8). This monument,like many others of this vintage, is at a mountaintop site because surveying viatriangulation required that surveyors see from observation site to target sites. Muchclearing and cutting of the native tropical vegetation was required to establish suchlines of site.

Table 11901–1903 to 1994–1995 triangulation-to-GPS site coordinates and velocities (mm/yr)reference frame of Saleh et al. (2004) into stable South American reference frame of We

Site number UTM Easting (m) UTM Northing (m)

1401 697,320.725 1,158,013.93912 691,921.263 1,184,264.08719 687,417.285 1,177,720.79733 649,646.863 1,184,753.4851400 682,525.776 1,149,352.1601031 711,453.292 1,160,871.73364 706,251.745 1,158,920.2661072 701,896.227 1,151,640.6161402 719,276.086 1,143,039.21369 675,480.817 1,181,151.60972 685,032.706 1,193,478.37882 717,353.232 1,138,577.35099 687,760.299 1,164,061.055115 647,095.405 1,116,505.047128 721,069.853 1,188,436.473135 684,975.457 1,121,672.391141 653,897.865 1,126,813.595145 672,645.901 1,142,738.965210 662,685.421 1,179,758.006321 676,093.278 1,150,405.627400 681,746.303 1,115,094.465472 702,286.063 1,129,010.591590 658,953.637 1,120,021.3458 684,916.475 1,181,330.0691267 659,772.687 1,182,905.970

Table 2Repeat GPS site velocities (mm/yr) and their uncertainties, from measurementsmade in 1994, 1998, 1999, and 2005, in stable South American reference frameupdated from Weber et al. (2001a) and discussed in text.

Site number Ve sVe Vn sVn

69 (Mt. Tabor) 17.95 1.43 2.17 1.21115 (La Fabiana) 3.51 1.23 1.40 1.11


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baselines. Collectively, 360 1994–1995 baselines, measuredbetween 94 monuments, with a total of 27 sites in common to the1901–1903 triangulation monuments, comprise our second epochGPS network. Some of these 1994–1995 GPS data were also pro-cessed using high-precision methods and comprise the first epochdata for our ongoing GPS-to-GPS measurements (see e.g., below;Weber et al., 2001a; Rodriguez et al., 2008).

To detect outliers, three-dimensional misclosures werecomputed for all triangles in the GPS network during preliminaryadjustments. Misclosures larger than 10 cm were used to identifyand remove low-quality baselines. The final GPS network was thenadjusted using a standard least-squares procedure (e.g., Leick,1990), fixing the 1994–1995 epoch ITRF96 coordinates of station115. The adjusted three-dimensional station positions were con-verted to geodetic coordinates referenced to the WGS84 ellipsoidwith an origin defined by the ITRF96 reference coordinates. Thesecoordinates were then projected onto the UTM projection plane.The variance–covariance matrix of the coordinates was propagatedthrough the conversion and projection, resulting in a completeerror matrix for the UTM coordinates. The resulting Northingsand Eastings of the UTM coordinates have standard deviations of�3–15 mm at 80 stations, �15–30 mm at 13 stations, anda maximum of �38 mm at station 590.

4. 1901–1995 Velocity estimation methods

We next estimated the 1901–1995 horizontal station velocitiesand uncertainties. The velocities are simply the differencesbetween the 1901–1903 and 1994–1995 horizontal station

with 20 � 3 mm/yr Ve added to transform velocities from fixed Northern Rangeber et al. (2001a).

Ve (mm/yr) sVe Vn (mm/yr) sVn

1.9 3.4 1.8 2.018.6 3.1 1.0 0.519.9 3.0 0.1 0.219.9 3.1 0.1 0.317.5 3.4 2.0 2.414.7 3.9 �11.0 2.218.0 3.7 2.3 2.2

9.1 3.7 �0.7 2.68.6 4.8 0.2 4.0

18.5 3.2 �1.5 0.717.0 3.1 0.9 0.911.7 4.8 3.4 4.218.7 3.3 2.7 2.112.8 5.2 �3.1 5.416.4 4.8 5.0 2.2

5.2 4.2 3.9 4.59.8 4.5 �4.9 4.47.5 3.5 �2.1 3.0

20.2 3.1 �0.1 0.416.4 3.4 �2.2 2.4

5.6 4.5 0.9 5.0�1.9 5.4 7.6 6.210.9 4.5 �1.5 4.920.0 3.0 �0.1 0.220.0 3.1 0.1 0.3



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coordinates (displacements), once in a common reference frame,divided by 92.5 years of time. To obtain such a common referenceframe, the 1901–1903 triangulation network was adjusted onITRF96, the datum of the 1994–1995 GPS network. This was doneby fixing several reference stations (>2 are needed) to their ITRF96values during the adjustment of the 1901–1903 triangulationobservations, while taking into account the full error matrix of thereference coordinates, and maintaining rigidity of the 1901–1903network in a least-squares sense. The solution that satisfies theseconditions is known as a free net solution (Brunner, 1979; Papo andPerelmuter, 1981; Koch, 1987), and may be viewed as the one thatminimizes the total energy stored in an analogous elastic network(Saleh, 2000). A standard free net solution is physically meaningfulonly if the reference stations chosen are mutually static betweenthe two surveys. In Trinidad, a large number of candidate combi-nations of reference stations exist and several possible active faultsmay exist. These factors make the search for mutually stablestations difficult, requiring many trial runs. To help automate thissearch, we followed Saleh (2000) and Saleh et al. (2004). Apreliminary displacement solution based on the adaptive free netapproach was computed using all of the 27 stations common to the1901–1903 and 1994–1995 networks as reference stations. Thisapproach modifies the datum definition iteratively, and graduallyreduces the influence of a station with large displacement or errorellipse on the datum definition. As the adjustment reaches the finaliteration, the datum definition is dominated by stations thatexperienced the least motion. This preliminary adjustmentuncovers all unstable and poorly observed and/or connectedcommon stations, as well as stations with large displacements. Theresulting displacement maps and error ellipses, together withgeologic insight, allow us to choose relatively static combinations ofreference stations. Once a certain combination is identified, it isused in a standard free net adjustment to define the coordinatesystem of the triangulation. In our final adjustment, stations 8, 33,210 and 1267 were chosen as reference stations (see Section 6.1).Following Balkaransingh (2001), Saleh et al. (2004) presentedpreliminary Trinidad 1901–1995 triangulation-to-GPS displace-ments in a fixed Northern Range reference frame, with a focus onhow the knowledge of these motions could be used to improve thenational geodetic framework. We expand greatly on the geophys-ical and geological aspects of the recovered velocity field here andpresent a more complete description of the methods used.

5. 1994–2005 Two-station repeat GPS data analysis

We collected episodic GPS data spanning w11 years at station115 (La Fabiana) in southern Trinidad and station 69 (Mt. Tabor) inTrinidad’s Northern Range to provide an independent constraint onthe overall motion across the island. Data were collected for 2–4days and w8–12 h per day in May–June 1994, May 1998, June 1999,and January 2005. These data were analyzed at the University ofMiami following the point positioning methods outlined in Weberet al. (2001a) and Dixon et al. (1997). We used the GIPSY softwaredeveloped at the Jet Propulsion Laboratory (JPL) and JPL satelliteephemeris and clock files (Zumberge et al., 1997). We first derivedsite velocities in IGSB00, which is approximately equivalent to theglobal ITRF-2000 reference frame (International Terrestrial Refer-ence Frame 2000) (Boucher et al., 2004). Robust site velocity errorswere then estimated following the strategy developed by Mao(1998), Mao et al. (1999), and Dixon et al. (2000). Following earlierworkers (e.g., Dixon et al., 1996; Dixon and Mao, 1997; DeMets andDixon, 1999; Weber et al., 2001a), we defined a stable SouthAmerican reference frame by fitting IGSB00 site velocities at tensites (FORT, KOUR, LPGS, PARA, UEPP, KOU1, CHPI, NEIA, LHCL,MPLA) to a rigid plate model (igsb00uSA ¼ �21.49, �123.08,


0.1089 � 0.0022). The small resulting mean residual (0.63 mm/yr)and low c2

v ¼ 2:16 indicate that these sites precisely defineda stable South American reference frame; this is the referenceframe in which we present the results from our repeat 1994–2005GPS measurements here. Weber et al. (2001a) had previouslypresented 1994–1998 velocities from these two Trinidadian sites(69 and 115) using five sites (ASC1, BRAZ, FORT, KOUR, LPGS) todefine a stable South American reference frame, but did not discussor analyze them in the context of the historic 1901–1995 geodeticcomparison or slip rate estimate, which we do here.

6. 1901–1995 Geodetic results

6.1. Velocity field

The estimation methods of the 1901–1995 velocities weredescribed in Section 4. The 1901–1903 ITRF96 coordinates of the 27common stations are subtracted from their 1994–1995 GPS coordi-nates. The preliminary displacement field revealed that severalstations (Stations 729, 63 ¼ 1031, 167, 472) experienced large,nonsystematic ‘‘displacements’’, and that stations 8,12,19, 33, 69, 72,210 and 1267 move as an ensemble and probably constitutea mutually stable set of fixed Northern Range reference stations,which could be used to define the datum in a final triangulationadjustment. Station 19 was excluded from the final reference stationensemble because it is near the on-strike El Pilar Fault extension intoTrinidad, which could in principal be a zone of active movement.Saleh et al. (2004) choose four of the Northern Range stationsensemble, 33, 1267, 210 and 8, as reference stations. The largestdeviation of the displacements of these stations from their mean is6 cm, and the largest azimuth deviation from their mean azimuth is5.2�. In addition, the distance from station 33 to station 8 is about40 km (Fig. 3B). This long distance contributes geometric strength tothe final solution, since the larger the area covered by the subset ofreference stations, the smaller the error ellipses are away from thereference stations. Trial solutions using different combinations ofNorthern Range reference stations had insignificant effects on theresulting final displacements. Stations that the preliminary adjust-ment uncovered as unstable, poorly observed, and/or weakly con-nected to the network, were not included in the final solution. Forexample, the large abhorrent displacement and error ellipse of point167 (see Fig. 3 in Saleh et al., 2004) disqualified this point.

Saleh et al.’s (2004) final displacement field was computedusing stations 8, 33, 210 and 1267 as fixed Northern Rangereference stations in a standard free net adjustment. As discussedearlier, these four reference sites are probably located withina rigid western and central Northern Range crustal block (e.g.,Weber et al., 2001b), north of the possibly active on-strike El PilarFault extension into Trinidad. However, the four referencestations are all located on the northern end of the island andtriangulation adjustment has the shortcoming that the uncer-tainties grow as a function of distance from the reference stations(Table 1; Figs. 3B and 5).

We start with the Saleh et al. (2004) 1901–1995 displacements,divide by 92.5 years of time, add a 20� 3 mm/yr eastward constantvelocity to each site velocity, including propagating velocity errors,to derive site velocities in Weber et al.’s (2001a) stable SouthAmerican reference frame and their errors (Fig. 3B; Table 1).

Stations 12, 69, and 72 were not used as reference stations, butare also located within the western and central Northern Range.These stations all move within�2–3 mm/yr of the reference stationmotions, consistent with our choice and treatment of the referencestations. The anomalous northeastward motion of station 128 inthe eastern Northern Range is most likely caused by a cornergeometric weakness in the triangulation data.


Fig. 5. Elastic dislocation fault model fits to 1901–1995 plate-motion-parallel (east)velocities. 20 � 3 mm/yr is added to all sites so northern sites move at full Caribbeanplate velocity of Weber et al. (2001a). Note the steep velocity gradient across the activeCentral Range Fault (CRF) located at center of profile. Best-fit CRF slip rate (shortdashed black line) is 12 � 2 mm/yr with a 2 km locking depth; solid black linerepresents 13 mm/yr fit with locking depth fixed at a more standard value of 10 km;best-fit two-fault model discussed in text is also shown by long dashed black line.F-tests indicate that c2 values obtained from best-fit wcreeping single-fault model(1–2 km locking depth) are indistinguishable at 95% significance from c2 values frommodels with a 10 km deep locked Central Range Fault and from a two-fault model thatputs an additional 2 mm/yr of slip on the Los Bajos Fault. Repeat GPS eastern velocitiesand uncertainties for sites 69 and 115 from Weber et al. (2001a) (filled circles) and thisstudy (open circles; Table 2), not used in these fits, are shown for comparison.


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The motions of stations 99, 1400 and 64 in the Northern (Caroni)Basin are within �1–3 mm/yr of those in the western and centralNorthern Range. This suggests that if there was any motion over thepast century across the on-strike El Pilar Fault extension, it musthave been small (see Section 6.1.2 below). Most stations south ofthe Central Range, however, move consistently from a few mm/yrto w13 mm/yr slower than the northern stations, and their veloc-ities systematically decrease southward. This pattern shows thatthe major gradient in the velocity field, and thus maximum strainrate accumulation, is dextral and occurs in the Central Range, on theCentral Range Fault, not across the eastward on-strike extension ofthe El Pilar Fault into Trinidad.

6.1.1. Fault-parallel displacement modelsTo obtain long-term fault strike-slip rates, we decomposed the

recovered velocities to fault-parallel and perpendicular compo-nents. We modeled the Central Range Fault-parallel velocities usingtwo single-fault elastic dislocation models, one locked and theother creeping (Savage and Burford, 1973). The parameters of eachmodel were obtained by a least squares adjustment, which best fitsthe model to the fault-parallel station velocities, and the statisticalsignificance of the estimated parameters was tested by a likelihoodratio significance (F) test (Koch, 1987).

Best-fitting the single locked fault model to the fault-parallelmotions we resolved a statistically significant far-field dextralvelocity of 11 � 3 mm/yr of locked far-field motion, with a statisti-cally insignificant 1.1 � 0.7 km locking depth. Neglecting velocitycovariances caused the best-fit rates to increase slightly to 13 mm/yrfor the locked fault model. The best-fit single creeping fault modelgave a statistically significant dextral 11 � 1 mm/yr creep rate. Wecannot resolve a Central Range Fault-normal shortening signal usingthese data (1.2� 1.2 mm/yr), but the numerical modeling of Giorgiset al. (2009) suggested that the current hilly topography in theCentral Range could indicate that 2 � 1 km of longer-termneotectonic contraction has been taken up across the fault.


Formal statistical tests cannot confirm that a locked CentralFault model fits our velocity data better than a creeping fault model.As stated above, the estimated looking depth is statisticallyinsignificant when compared to its estimated error. We also usedan F-ratio test (e.g., Stein and Gordon, 1984) to evaluate of whethera locked fault model improves the fit over using the simplercreeping fault model. When the correlations between the esti-mated velocities were ignored, the F-ratio statistic was 5.613 (forthe creeping model, c2 ¼ 82.50, N ¼ 25, p1 ¼ 1; for the lockedmodel, c2 ¼ 58.89, N ¼ 25, p2 ¼ 2). The tabulated value for F1,23 is4.28 for a confidence level of 95% and 7.88 for 99%. Thus, althoughthe test reveals a better fit for the locked fault at 95% confidence,this conclusion is reversed at 99% confidence. The same test wasthen applied to a subset of 16 stations, which appeared to be moststable (e.g., 1031 was excluded for its large N–S velocity componentas were 82, 472, 135, etc.). The full error matrix of the 16 fault-parallel velocities was taken into account. The resulting F-ratiostatistic was 0.596 (for the creeping model, c2 ¼ 27.21, N ¼ 16,p1 ¼ 1; for the locked model, c2 ¼ 28.42, N ¼ 16, p2 ¼ 2), which ismuch smaller than the tabulated F1,14 for the 99% confidence (8.86),95% (4.6) and even 90% confidence (3.1). Therefore, our geodesy-derived 1901–1995 velocities alone cannot confirm that the CentralRange Fault is locked.

6.1.2. Bounds on horizontal motion across on-strike El Pilar Faultextension

To quantify the magnitude of possible horizontal movementalong the El Pilar Fault’s on-strike eastward extension into Trinidad,we fit simple step functions (but implying no specific faultbehavior) to the E–W and N–S motions across this zone. Themotions of stations 8, 33, 210 and 1267 were fixed to zero whereasthose of 64, 99, 1400 and 1401 were treated as unknowns, equal tothe height of the step. This sub-network spans w35 km of fault-perpendicular distance, probably enough to resolve motions relatedto either a creeping or locked on-strike El Pilar Fault extension. Thefull velocity covariance matrix was used in the least-squares fitting.The E–W motion resolved was 2.2 � 1.8 mm/yr (dextral); theresolved N–S motion was 2.7 � 2.0 mm/yr (shortening). Based onthe likelihood ratio test, both are statistically insignificant. Thus, ifany dextral strike-slip occurred along the on-strike El Pilar Faultextension in Trinidad during the last wcentury, its rate was too lowto accommodate a significant fraction of the total plate motion, andit is likely not the active strike-slip fault in Trinidad today.

6.1.3. Plate-motion-parallel modelsIn addition to the sharp Central Range Fault geomorphic linea-

ment we observed and discussed above and show in Fig. 3A, the LosBajos Fault is also marked by a sharp topographic lineament, sug-gesting that it too may be an active fault (Fig. 3A). Synthesis ofpetroleum-industry-generated geologic data has previously shownthat the Los Bajos Fault is an important Pleistocene or youngerdextral strike-slip fault offshore in the Gulf of Paria (Tyson, 1989),and a significant Neogene on-land structure (Wilson, 1940).Paleoseismic and geomorphic studies have reported that the LosBajos Fault cuts Holocene sediments, and its pronounced but subtlegeomorphic expression was used to suggest that it probablyaccommodates slow (a few mm/yr) strike-slip (Crosby et al., 2001;Dames and Moore Report, unpublished, 1996; J. Hengesh, 1999,personal communication). Active distortion of oil well boreholesnear the Los Bajos Fault is also consistent with current activity(Harper and Chambers, 2004). However, new trenching data thatwe collected (Prentice and Weber, unpublished, 2005) do notunequivocally confirm this interpretation.

To test for multiple active strike-slip faults in Trinidad, and tofurther explore the magnitude of plate-motion-related shear taken



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up across the Central Range Fault, we next best-fit elastic disloca-tion models to wplate-motion-parallel (east) velocities. Thevelocities were plotted along a profile normal to the on-strike ElPilar extension, the Central Range Fault, and the Los Bajos Fault(Fig. 5). Plate-motion-parallel (east) velocities were used becausethese faults are all oblique to one another, as well as oblique to platemotion (Weber et al., 2001a). We neglect velocity covariances in theinversions that follow.

First, we fit a single-fault model using two adjustable parameters,obtaining a best fit (c2 ¼ 48.52; c2

v ¼ 1:9) with 12 mm/yr of slip onthe Central Range Fault, a 2 km locking depth, and 8 mm/yr of motiontaken up in southern Trinidad (Fig. 5). We then fix the locking depthto 10 km, a more standard value than the shallow locking depth andget a best fit (c2¼ 58.00; c2

v ¼ 2:3) with 13 mm/yr on Central RangeFault. It is also clear to see in Fig. 5 that site velocities along thenorthern and central parts of this velocity profile were systemati-cally underestimated when we modeled 13 mm/yr of locked motion(10 km locking depth) on the on-strike extension of the El Pilar Fault.The F-ratio test confirmed this conclusion. For the Central RangeFault solution with 10 km locking depth, the c2¼58.0, the number ofobservations N ¼ 25 and number of parameters p ¼ 1, while for thesolution that does not constrain the locking depth, c2¼48.52, N¼25and p ¼ 2. To test whether constraining the locking depth at 10 kmfits our velocities better, the corresponding F-ratio statistic is 3.923.The tabulated value for F1,24 is 7.82 at the 99% confidence level and4.26 at the 95% confidence level. This implies that constraining thelocking depth to 10 km does not improve the fit over floating it andallowing it to be resolved by the data (as a shallow 2 km depth).

We next attempt to resolve strike-slip motion across the Los BajosFault. Only a single station, 115, exists southwest of this fault. Fixingthe locking depth to 10 km and fitting a two-fault model gives a bestfit with 14 mm/yr on the Central Range Fault, 2 mm/yr on the LosBajos Fault, 6 mm/yr in the southern offshore, with a c2 of 58.12.According to the likelihood ratio F-test (Koch, 1987), the two-faultmodel does not improve the fit over a single locked fault model with10 km locking depth, which, in turn, did not improve the fit over ourbest-fit single locked fault model. We are therefore unable to resolveany significant Los Bajos strike-slip motion geodetically. The sharpLos Bajos Fault scarp is, however, consistent with strike-slip motionat a level below that which we can currently resolve using these data(w�5 mm/yr), or perhaps with this approximate level of dip-slip oroblique-slip motion. We should be able to test for �5 mm/yr-levelmotions using the newer, higher-precision GPS data that we arecurrently acquiring (Rodriguez et al., 2008).

7. Repeat GPS results

Repeat 1994–1999 GPS velocities at sites 69 and 115, are given inthe stable South American reference frame defined in Weber et al.(2001a), shown in Fig. 3B, and given in Table 2. Station 69 is roughly35 km north of the Central Range Fault; it moves 18.1 � 1.4 mm/yrtoward 083� � 4�. Within 2s uncertainty, this site moves at the fullCaribbean plate rate of w20 mm/yr predicted by Weber et al.(2001a); at 1s uncertainty it moves a few mm/yr slower. Site 115,about 60 km away in southern Trinidad, moves at 3.8 � 1.2 mm/yrtoward 68� � 17�, 14 � 3 mm/yr slower in its east velocitycomponent than station 69. This difference level is a robust result,precisely that obtained by Weber et al. (2001a). Figs. 3B and 5 showthat these repeat GPS-to-GPS velocities are consistent with ourhistorically derived (1901–1995) velocities. In addition, althoughnot used in our velocity inversions, the 14 � 3 mm/yr differencecompares favorably with our historic (1901–1995) slip rate esti-mates. Site 69’s velocity was underestimated when we modeledw13 mm/yr of slip on the on-strike El Pilar Fault extension.The new GPS-to-GPS velocities for these two sites are both


systematically w2 mm/yr slower than Weber et al.’s (2001a) esti-mates that used less data and a slightly different reference frame.

8. Tectonic geomorphology

The geodetically discovered, on-land, active Central Range Faultcoincides with a major topographic lineament that consists oflinear drainages, aligned topographic saddles and troughs, offsetridges, right-laterally deflected streams, and linear scarps (Crosbyet al., submitted for publication; Prentice et al., submitted forpublication) (Fig. 3A). Paleoseismology trenching at the Samlal-singh trench site (Fig. 3A) clearly demonstrated that this fault cutsw500–5000-year-old Holocene sediment; these data can beinterpreted as indicating that the fault has ruptured at least oncewithin the last w5000 years (Prentice et al., submitted for publi-cation). Soto et al. (2007) used shallow industry seismic reflectiondata to show that the active on-land Central Range Fault extendsinto Trinidad’s eastern offshore where it dextrally offsets a nowunderwater, probable Quaternary fluvial channel.

A number of broad-scale tectonic geomorphic features alsosupport that the Venezuelan El Pilar Fault steps southward acrossan active pull-apart basin in the Gulf of Paria to the Central RangeFault in Trinidad. Based on qualitative east-to-west differences innorthern Trinidad’s coastal and alluvial fan morphology, Weber(2005) proposed that western Trinidad is actively subsiding into theGulf of Paria pull-apart basin. These geomorphic features aresummarized and shown in Fig. 3A. In the eastern portion of theNorthern Range, the entire coastline is sharp and emergent.Subaerial exposures of young (w40–137 ka) coastal terraces atelevations of w15 m above sea level are present in northeasternTrinidad (Weber and Ritter, 2007). Across a ‘‘hinge line’’ in west-central Northern Range, the coastline becomes highly scalloped,drowned, and submergent. Trinidad’s western islands are probablyhighly sunken and drowned former Northern Range peaks. Ineastern Trinidad, around the village of Valencia, Northern Range-front Quaternary alluvial fans are highly incised by modernstreams, likely reflecting that they are elevated relative to local baselevel. In northwestern Trinidad, probable correlative range-frontfan deposits are buried in the subsurface of the sunken Caroni Basin(Caroni swamp). These buried western gravels have been exten-sively drilled, and are an important local source of groundwater.

Ritter and Weber (2007) evaluated the Weber (2005) modelusing quantitative drainage basin morphometry data from 11drainage basins in the western Northern Range and 29 drainagebasins in the eastern Paria Peninsula, Venezuela. Spatial trends inmean drainage elevations, outlet elevations, and basin asymmetry,are all consistent with symmetric sinking and tilting into the Gulf.Long wavelength, low amplitude flexural uplift along the flanks ofthe pull-apart basin might explain the subaerially exposed coastalterraces observed in northeastern Trinidad, although other causesshould also be considered, e.g., aggressive stream incision in therainy eastern (windward) side of the island that results in surfaceuplift a la Molnar and England (1990).

9. Discussion and conclusions

Analyses of our 1901–1995 and 1994–1999 geodetic velocityfields show that w2/3 of the total 20 mm/yr Caribbean–SouthAmerican dextral shear (Weber et al., 2001a) is taken up across theCentral Range Fault in Trinidad, which was, prior to the geodeticwork presented here and earlier (Weber et al., 2001a), an unrecog-nized active on-land fault. This discovery stimulated recent onshore(Crosby et al., submitted for publication; Prentice et al., submitted forpublication) and offshore (Soto et al., 2007) paleoseismic research.Many older maps and models that put maximum straining on the



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on-strike eastward extension of the El Pilar Fault or assumed thatpresent-day oblique plate convergence is still occurring requirerethinking. Robertson and Burke (1989) used geological data to inferthat the Caribbean–South American plate boundary zone is severalhundred kilometers wide near Trinidad. The geodetic observationspresented above, however, suggest that the Central Range Faultaccommodates w65% of Caribbean–South American plate motion inTrinidad, and that displacement in the plate boundary zone iscurrently more localized than previously thought.

The geodetically discovered, on-land, active Central Range Faultcoincides with a major topographic lineament (Fig. 3A). Paleo-seismic studies show that the fault cuts Holocene sediment andconfirm that it is active (Crosby et al., submitted for publication;Prentice et al., submitted for publication). Soto et al. (2007) showedthat the Central Range Fault extends into Trinidad’s eastern offshoreand that it dextrally offsets a now-underwater Quaternary fluvialchannel. In addition, broad-scale geomorphic features in northernand central Trinidad support that Venezuela’s El Pilar Fault steps tothe south across an active Gulf of Paria pull-apart basin to the CentralRange Fault in Trinidad (Weber, 2005; Weber and Ritter, 2007).

Our 1901–1995 triangulation-to-GPS analysis resolveda 12� 3 mm/yr Central Range Fault-parallel slip rate. Repeat (1994–2005) GPS measurements also resolved a comparable 14� 3 mm/yrof total dextral eastward shear across Trinidad. Balkaransingh(2001) compared 96 of the 1901–1903 triangulation-derived posi-tions used in this study to 1963 triangulation–trilateration posi-tions. Her 1901–1963 displacements gave a comparable, dextral,best-fit, fault-parallel, elastic slip rate across the Central RangeFault, of 13 � 3 mm/yr.

The question of a locked versus creeping Central Range Fault isan extremely important issue, with significant implications forseismic risk, durability of the existing petroleum infrastructure, andthe planning of future industrial and national infrastructure inTrinidad. This study was initiated, in part, as an attempt to beginaddressing this still open debate. Our best-fit single-fault modelgave a shallow (w1–2 km) locking depth that essentially suggestsfault creep. Statistical tests indicate that setting the locking depth to10 km does not improve the fit to our data. Thus, using just thegeodetic data, we cannot discriminate between the creeping andlocked fault possibilities.

Several other lines of evidence, however, do suggest that theCentral Range Fault could be locked. First, creeping strike-slipfaults are, in general, rare. Strike-slip creep has been documentedin only a few locations, e.g., along the central portion of the SanAndreas Fault and along a few subsidiary faults like the Haywardand Calavaras Faults (e.g., Lienkaemper et al., 2001). Evidence forcreep has been sought after along other strike-slip faults in Japanand New Zealand with negative results (e.g., Scholtz, 1990, andreferences therein). A characteristic feature of the creeping Cal-ifornia faults is that they have very frequent small earthquakes andare thus easy to identify seismically. Fig. 2, created using data fromthe nearby Venezuelan seismic network, does not show thispattern. A complete catalogue from the local Trinidad seismicnetwork, which has been operating since 1953, plotted as maps(e.g., Latchman, 1998, Figs. 5, 7), also shows no clustering of smallmagnitude events on the Central Range Fault. We do not know theperformance characteristics of this network; it is possible thatmicroseismicity related to creep goes undetected, or that a clus-tering pattern could result from better event locations. One of us(Weber) did an extensive direct search for offset cultural featuresthat would mark creep on the Central Range Fault from Pointe-a-Pierre to the Navet Dam and did not find any such evidence.Trinidad’s European inhabitation began in 1498 with Columbus’second landing in the New World. Trinidad’s historic record iscomplete to w1800 (Robson, 1964). Historic earthquakes along the


Central Range Fault cannot be ruled out, but they are not unam-biguously required. Paleoseismology shows that the Central RangeFault could have ruptured twice in the past w5000 years (Crosbyet al., submitted for publication; Prentice et al., submitted forpublication). These observations and inferences lead us to suspectthat the Central Range Fault might be locked rather than creeping. Ifso, it could constitute a major seismic risk for Trinidad’s populationand petroleum and national infrastructure. We are faced with theimportant task of resolving this dilemma. We hope to be able tofurther address this important and unresolved issue using new datafrom ongoing high-precision GPS surveys (e.g., Rodriguez et al.,2008), but establishing and monitoring several creep arrays thatcross the fault might be the quickest and most direct way to do this.

Although geomorphic, paleoseismic, and borehole distortiondata (e.g., Crosby et al., 2001; Harper and Chambers, 2004) suggestthat a few mm/yr of dextral-slip is taken up across the Los BajosFault, we cannot confirm this geodetically. The w15 mm/yr ofmotion that we get from our elastic dislocation modeling andrepeat GPS-to-GPS measurements, differenced from Weber et al.’s(2001a) 20 mm/yr of total plate motion, suggest that w5 mm/yr ofmotion could be taken up across the Los Bajos Fault or on additionalfaults along the south coast or in the offshore south of Trinidad(Columbus channel). Again, our ongoing high-precision GPSsurveys (e.g., Rodriguez et al., 2008) should help us resolve some ofthese details. This is of significance since the bulk of Trinidad’spetroleum industrial activity is concentrated in southern Trinidad.

The presence of an active strike-slip fault in central Trinidad haspotential long-term geologic implications that also merit somediscussion. The N70�E-trending Central Range Fault is oblique toN86� � 2�E directed Caribbean–South American plate motion (Weberet al., 2001a). Thus, the style of active deformation that we mightexpect along the fault is transpressional (e.g., Sanderson andMarchini, 1984). Using geodesy, we could not resolve any CentralRange Fault-normal shortening (1.2 � 1.2 mm/yr). According to thenumerical modeling of Giorgis et al. (2009), however, the hillyCentral Range topography could reflect active surface uplift as anisostatic response to 2 � 1 km of neotectonic shortening andassociated crustal thickening. Alternatively, much of the rockexhumation (where rocks as old as Lower Cretaceous are exposed)and folding and thrusting observed in the Central Range could berelated to fossil strain that accumulated during pre-10 Ma Carib-bean–South America oblique convergence (e.g., Pindell et al., 1998),not during the current strike-slip regime. This, too, is an importantopen issue waiting to be resolved.

Acknowledgements

We thank those who helped with fieldwork and logistics duringrecovery of the 1901–1903 triangulation monuments and subse-quent GPS measurements, including Godfrey Almoralaz, Glen andKareem Decoteau, Murchison Pierre, Keith Rowley, John Scott,Maisha Jeffers, Richard Oliver, Lloyd Lynch, Kelvin King, MichaelSoo Ting, Dexter Davis, Kamal Sant, Joe Barbaste, Clinton Stewart,L. Walker, Anthony Gonzales, Wilkie Balgobin, Wayne and StephenWilliams, Dean Childs, Brennan O’Neill, Steve Fisher, Ruth Neilan,Richard Robertson, Lloyd Lynch, Cpl. Edward Nicholls and addi-tional Defense Force soldiers, Andrew McCarthy, John Sakolosky,Mark Bordelon, Bobby Rhamdani, Bob Speed, Raid al-Tahir, MarkWeber, and Previn Kennedy. We also thank Lands and Surveyspersonnel at the Red House in Port-of-Spain who helped us accessthe original hand-written triangulation data log books and surveysummary (Smart, 1903), and shared their busy Xerox machine. Weare grateful to the International Geodynamics Service (IGS) andother members of the international geodetic community formaking high-quality data from permanent GPS stations publicly



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available, and to FUNVISIS for providing local earthquake data usedfor construction of Fig. 2. This work was supported by the CaltechPresident’s Fund, NSF Postdoctoral Fellowship EAR-9404214, BHPBilliton, and a Conoco (DuPont) Young Professor’s Award to Weber;Weber thanks Judith Hannah, Keith James, Tomas Villamil, KarenTindale, and James Deckelman for logistical support.

References

Algar, S., Pindell, J., 1994. Structure and deformation history of the Northern Rangeof Trinidad and adjacent areas. Tectonics 12, 814–829.

Babb, S., Mann, P., 1999. Structural and sedimentary development of a Neogenetranspressional plate boundary between the Caribbean and South Americanplates in Trinidad and the Gulf of Paria. In: Mann, P. (Ed.), Caribbean Basins.Elsevier, Amsterdam, pp. 495–557.

Balkaransingh, S., 2001. The tectonic displacement field in Trinidad and Tobagoduring the 20th century. M.Phil. Thesis. Department of Surveying and LandInformation, University of the West Indies, St. Augustine, Trinidad.

Bomford, G., 1980. Geodesy, fourth ed. Oxford Clarendon Press, Oxford.Boucher, C., Altamimi, Z., Sillard, P., Feissel-Vernier, M., 2004. The ITRF2000. Inter-

national Earth Rotation and Reference Systems Service (IERS). In: IERS TechnicalNote 31. Verlag des Bundesamtes fur Kartographie und Geodasie, Frankfurt amMain, Germany. Online version at. http://www.iers.org/iers/publications/tn/tn31/.

Brunner, F., 1979. On the analysis of geodetic networks for the determination of theincremental strain tensor. Survey Review April XXV (192).

Crosby, C., Prentice, C., Weber, J., Hengesh, J., 2001. Paleoseismic and geomorphicevidence for quaternary fault slip on the central range and Los Bajos faults,South American–Caribbean plate boundary, Trinidad. In: Transactions: TenYears of Paleoseismology. International Lithosphere Project, Paleoseismology,Kaikoura, New Zealand.

Crosby, Christopher J., Prentice, Carol S., Weber, John C., Ragona, Daniel. Logs ofPaleoseismic Excavations Across the Central Range Fault, Trinidad. US Geolog-ical Survey Open-File Report, submitted for publication.

DeMets, C., Dixon, T., 1999. New kinematic models for Pacific–North Americamotion from 3 Ma to present: I. Evidence for steady motion and biases in theNUVEL-1A model. Geophysical Research Letters 26, 1921–1924.

DeMets, C., Gordon, R.G., Argus, D.F., Stein, S., 1990. Current plate motions.Geophysical Journal International 101, 425–478.

DeMets, C., Gordon, R.G., Argus, D.F., Stein, S., 1994. Effect of recent revisions to thegeomagnetic time scale on estimates of current plate motions. GeophysicalResearch Letters 21, 2191–2194.

Deng, J., Sykes, L.R., 1995. Determination of an Euler pole for contemporary relativemotion of the Caribbean and North American plates using slip vectors ofinterplate earthquakes. Tectonics 14, 39–53.

Dixon, T.H., Mao, A., 1997. A GPS estimate of relative motion between North andSouth America. Geophysical Research Letters 24, 535–538.

Dixon, T.H., Mao, A., Stein, S., 1996. How rigid is the continental interior of the NorthAmerican plate? Geophysical Research Letters 23, 3035–3038.

Dixon, T.H., Mao, A., Bursik, M., Heflin, M., Langbein, J., Stein, R., Webb, F., 1997.Continuous monitoring of surface deformation at Long Valley Caldera with GPS.Journal of Geophysical Research 102, 12017–12034.

Dixon, T.H., Miller, M., Farina, F., Wang, H., Johnson, D., 2000. Present-day motion ofthe Sierra Nevada block and some tectonic implications for the Basin and Rangeprovince, North American Cordillera. Tectonics 19, 1–24.

Doser, D., Van Dusen, S., 1996. Source processes of large earthquakes in the SECaribbean (1929–1960). Pure and Applied Geophysics 146, 43–66.

Edwards, K., 1999. Computation of the precise geoid of Trinidad and Tobago. M.Phil.Thesis. Department of Surveying and Land Information, University of the WestIndies, St. Augustine, Trinidad.

Flinch, J., Rambaran, V., Ali, W., De Lisa, V., Hernandez, G., Rodrigues, K., Sams, R.,1999. Structure of the Gulf of Paria pull-apart basin (eastern Venezuela–Trinidad). In: Mann, P. (Ed.), Caribbean Sedimentary Basins. Elsevier,Amsterdam, pp. 477–494.

Giorgis, S., Tang, J., Sirianni, R., 2009. Constraining neotectonic orogenesis using anisostatically compensated model of transpression. Journal of Structural Geology.doi:10.1016/j.jsg.2009.02.003.

Grant, I., 1996. Re-adjustment of the 1901–1903 triangulation of Trinidad –mathematical aspects. B.Sc. Thesis. Department of Surveying and Land Infor-mation, University of the West Indies, St. Augustine, Trinidad.

Harper, T.R., Chambers, J.L., 2004. Stress state and its influence on drilling perfor-mance in the Brighton Marine Field, Trinidad. Marine and Petroleum Geology21 (7), 947–963.

King K., 1996. Re-adjustment of the 1901–1903 triangulation of Trinidad – datamanagement and programming aspects. B.Sc. Thesis. Department of Surveyingand Land Information, University of the West Indies, St. Augustine, Trinidad.

Koch, K.R., 1987. Parameter Estimation and Hypothesis Testing in Linear Models.Springer Verlag, Berlin.

Kreemer, C., Holt, W., Haines, J., 2003. An integrated global model of present-dayplate motions and plate boundary deformation. Geophysical Journal Interna-tional 154, 8–34.


Kugler, H., 1960. Geological Map of Trinidad. Orell Fussli Arts Graphiques S.A.,Switzerland.

Latchman, J., 1998. Seismic potential of the SW Tobago fault system. M.Phil. Thesis.University of the West Indies, St. Augustine, Trinidad.

Leick, A., 1990. GPS-Satellite Surveying. Addison Willey, NY.Lienkaemper, J.L., Galehouse, J.S., Simpson, R.W., 2001. Long-term monitoring of

creep rate along the Hayward fault and evidence for a lasting creep responseto 1989 Loma Prieta earthquake. Geophysical Research Letters 28 (11),2265–2268.

Lopez, A.M., Stein, S., Dixon, T., Sella, G., Calais, E., Jansma, P., Weber, J., LaFemina, P.,2006. Is there a northern Lesser Antilles forearc block? Geophysical ResearchLetters 33, L07313. doi:10.1029/2005GL025293.

Mao, A., 1998. Geophysical applications of the global positioning system. Ph.D.Thesis. Miami, Florida, University of Miami, 149 pp.

Mao, A., Harrison, C.G.A., Dixon, T.H., 1999. Noise in GPS coordinate time series.Journal of Geophysical Research 104, 2797–2816.

Mendoza, C., 2000. Rupture history of the 1997 Cariaco, Venezuela, earthquakefrom teleseismic P waves. Geophysical Research Letters 27 (10), 1555–1558.

Molnar, P., England, P., 1990. Late Cenozoic uplift of mountain ranges and climatechange: chicken or egg? Nature 346, 29–34.

Papo, H.B., Perelmuter, A., 1981. Datum definition with free net adjustment. Bulle-tine Geodesique 55, 218–226.

Perez, O., Bilham, R., Bendick, R., Velandia, J., Moncayo, C., Hoyer, M., Kozuch, M.,2001. Velocity field across the southern Caribbean plate boundary and estimatesof Caribbean/South American plate motion using GPS geodesy 1994–2000.Geophysical Research Letters 28 (15), 2987–2990.

Pindell, J., Higgs, R., Dewey, J., 1998. Cenozoic palinspastic reconstruction, paleogeo-graphic evolution, and hydrocarbon setting of the northern margin of SouthAmerica. In: Pindell, J., Drake, C. (Eds.), Paleogeographic Evolution and Non-glacialEustasy, northern South America. SEPM Special Publication No. 58, pp. 44–85.

Prentice, C., Weber, J., Crosby, C., Ragona, D. Central Range fault, Trinidad: paleo-seismic studies along the Caribbean–South American plate boundary. Geology,submitted for publication.

Ritter, J., Weber, J., 2007. Geomorphology and Quaternary geology of the NorthernRange, Trinidad and Paria Peninsula, Venezuela: recording quaternary subsi-dence and uplift associated with a pull-apart basin. In: Proceedings, GeologicalSociety of Trinidad and Tobago, Fourth Geological Conference.

Robertson, P., Burke, K., 1989. Evolution of the southern Caribbean plate boundary,vicinity of Trinidad and Tobago. American Association of Petroleum GeologistsBulletin 73, 490–509.

Robson, G.R., 1964. An earthquake catalog for the Caribbean, 1530–1960. Bulletin ofthe Seismological Society of America 54, 785–832.

Rodriguez, A., Weber, J., Schmalzle, G., 2008. Global Positioning System (GPS)determination of motions, neotectonics, and seismic hazard in Trinidad andTobago. American Geophysical Union Annual Meeting (abstract).

Russo, R.M., Speed, R.C., 1992. Oblique collision and tectonic wedging of the SouthAmerican continent and Caribbean terranes. Geology 20, 447–450.

Russo, R.M., Speed, R.C., Okal, E.A., Shepherd, J.B., Rowley, K.C., 1993. Seismicity andtectonics of the southeast Caribbean. Journal of Geophysical Research 98,14229–14319.

Saleh, J., 2000. Robust estimation based on energy minimization principals. Journalof Geodesy 74 (3–4), 291–305.

Saleh, J., Edwards, K., Barbaste, J., Balkaransingh, S., Grant, D., Weber, J., Leong, T.,2004. On some improvements in the geodetic framework of Trinidad andTobago. Survey Review 37, 604–625.

Sanderson, D.J., Marchini, W.R.D., 1984. Transpression. Journal of Structural Geology6, 449–458.

Savage, J.C., Burford, 1973. Geodetic determination of relative plate motion incentral California. Journal of Geophysical Research 78 (5), 832–845.

Scholtz, C.H., 1990. The Mechanics of Earthquakes and Faulting. CambridgeUniversity Press, 439 pp.

Sella, G., Dixon, T., Mao, A., 2002. REVEL: a model for recent plate velocities fromspace geodesy. Journal of Geophysical Research 107 (B4), 2081. doi:10.1029/2000JB0000033.

Sillard, P., Altamini, Z., Boucher, C., 1998. The ITRF96 realization and its associatedvelocity field. Geophysical Research Letters 25, 3223–3226.

Smart, E.R., 1903. Unpublished Survey Notes, Red House, Lands and Surveys Divisionof Trinidad and Tobago. Post-of-Spain, Trinidad.

Soto, M.D., Mann, P., Escalona, A., Wood, L.J., 2007. Late Holocene strike-slip offset ofa subsurface channel interpreted from three-dimensional seismic data, easternoffshore Trinidad. Geology 35 (9), 859–862.

Speed, R.C., 1985. Cenozoic collision of the Lesser Antilles arc and continental SouthAmerica and the origin of the El Pilar fault. Tectonics 4, 41–69.

Speed, R., Russo, R., Weber, J., Rowley, K., 1991. Evolution of southern Caribbeanplate boundary, vicinity of Trinidad and Tobago: discussion. American Associ-ation of Petroleum Geologists Bulletin 75, 1789–1794.

Stein, S., Gordon, R.G., 1984. Statistical tests of additional plate boundaries fromplate motion inversions. Earth and Planetary Science Letters 69, 401–412.

Tyson, L., 1989. Structural features associated with the Los Bajos fault and theirinterpretation in light of current theories of strike-slip tectonics. Transactionsof the 12th Caribbean Geological Conference, St. Croix, U.S. Virgin Islands,pp. 403–414.

van Andel, T., Sachs, P., 1964. Sedimentation in the Gulf of Paria during the Holocenetransgression; a subsurface acoustic reflection study. Journal of MarineResearch 22 (1), 30–50.


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ARTICLE IN PRESS

Weber, J.C., 2005. Neotectonics in the Trinidad and Tobago, West Indies Segment ofthe Caribbean–South American Plate Boundary, vol. 204. Geological Institute ofHungary, pp. 21–29. Occasional Papers.

Weber, J., Dixon, T., DeMets, C., Ambeh, W., Jansma, P., Mattioli, G., Saleh, J., Sella, G.,Bilham, R., Perez, O., 2001a. A GPS estimate of Caribbean–South American platemotion, and geologic implications for Trinidad and Venezuela. Geology 29, 75–78.

Weber, J., Ferrill, D., Roden-Tice, M., 2001b. Calcite and quartz microstructural geo-thermometry, Northern Range, Trinidad. Journal of Structural Geology 23, 93–112.


Weber, J., Ritter, J., 2007. Northern range: structures, metamorphism, and tectonicgeomorphology. In: Geological Society of Trinidad and Tobago, FourthGeological Conference, Guidebook, Field Trip #2, 28 pp.

Wilson, C.C., 1940. The Los Bajos fault of south Trinidad, British West Indies.Bulletin of American Association of Petroleum Geologists 24, 2102–2125.

Zumberge, J., Heflin, M., Jefferson, D., Watkins, M., Webb, F., 1997. Precise pointpositioning for efficient and robust analysis of GPS data from large networks.Journal of Geophysical Research 102, 5005–5017.


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